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the calibration values for the NXT sensor are set in the factory and seem to be different for each sensor (which implies that there must be a reasonable amount of variation between different sensors). On the NXT the RGB values are true raw values and the calibration adjustments used to identify Lego colors are made in the leJOS driver.

But on the EV3 there is no calibration, and the sensor RGB mode

is not well documented and was not exposed at all in the first version of the Lego software...So the bottom line is I doubt if anyone outside of Lego (and maybe NI) knows if the RGB values are raw sensor readings or if any sort of calibration data has been applied

So it doesn't sound like you will have a consistent set of data anyway - I'd suggest just seeing what coloured balls you can get cheaply and just trying them.